This invention relates to additives for metallurgic liquids, which are effective to improve the structural and mechanical characteristics of metal or metal alloy articles of manufacture. In particular, this invention is concerned with additives effective to pick up and remove slag and degassing reaction products having non-metallic character from metallurgic liquids.
It is known that a problem of metallurgic processes is that of removing the non-metallic particles which are to be found or form in the course of metal melting and re-melting, which particles, if retained as inclusions in metal articles produced thereby would break the regularity of the crystal lattice of the metallic bond, and constitute a weak point which degrades the mechanical characteristics of the final product.
The means and systems currently employed to obviate the problem of removal of such particles or generic slag vary with the types of metallurgy, and have degrees of effectiveness ranging from none to limited, providing somewhat erratic results.
Thus, as an example, in reduction metallurgy, liquid slag flows out of the furnace, or blast furnace, together with the metals and metal alloys obtained by reduction of their minerals. The lower or higher density of slag relatively to the metals and alloys is the only natural way of causing some, never all, of the slag to float on or decant from the metal mass. In the remelting metallurgy, such as is typical with steelworks, that process is carried out under an artificially formed liquid slag which is only partly removed by draining it from the furnace crucible into suitable tanks. Thus, solidified steel will contain in the form of inclusions, in addition to endogenous inclusions, also residues of the slag retained in the liquid bath.
In the steel refining process, even where argon gas jets are used to encourage floating up the exogenous and endogenous inclusions, the results are only moderately satisfactory and quite erratic.
In the manufacture of cupola furnace cast iron, or induction furnace cast iron, only the slag having a high-melting point and clotted slag will surface in the ladle on casting and can be readily removed. Slag having a low-melting point, which remains liquid, such as the compounds of fayalite, wustite, tridymite+fayalite or fayalite+wustite whose formation is a recurring phenomenon in cast iron melting, are left homogeneously admixed to metal in casting. Solid state cast iron may, therefore, periodically exhibit in its structure cast fragments of the macroinclusions which can be detected at the optical microscope in the form of dark spots of an irregular geometric appearance.
Where the presence of liquid slag is instead conspicuous, the latter will, on account of its density, during the solidification stage of cast iron, interpose itself at the upper regions between the agglutinant and solid metal of the cast piece. Thus, the latter is bound to undergo outer structural deformations resulting in a reject.
In remelting non-ferrous metals and alloys, removal of the bound slag having a high-melting point is effected by skimming the liquid surface in the ladle, but similarly to cast iron, liquid slag having a low-melting point cannot be removed. In particular with aluminum and its alloys, inclusions of alumina are recurrent, more likely aluminum hydroxides, which although forming slag having a high-melting point, fail to surface and result in highly objectionable inclusions if incorporated to the solid formed. This phenomenon already appears in processing aluminum metal and requires, for a partial removal thereof, expensive filtering processes. With copper and its alloys, the low-melting liquid slag comprises non-stoichiometric compounds wherein sulphides, phosphides, etc. prevail.
Besides the foregoing, which adopts a continuous character which may be visually apparent, intermittent phenomena also occasionally occur in the manufactures of all ferrous and non-ferrous alloys, such as an increase, without any apparent reasons or chemical changes, in the viscosity of the molten alloy baths, which affects castability. This phenomenon is to be attributed to the formation of colloidal phases within the liquid alloy, which phases are sometimes inherent to addition alloys. Such colloids are formed by combining elements having a high electronegativity, with elements having a high electropositivity in a non-stoichiometric form, and occasionally assume, during solidification of the alloys, layered films which, in some cases intervene between the crystal lattices of the metallic bond, and in other cases assume the form of long filaments which extend along the edges of the grains, and have shown to be the cause or joint cause of the fractures detected in articles formed from such alloys.